The braconid wasp Cotesia vestalis (Haliday) is a primary larval endoparasitoid of the diamond-back moth, Plutella xylostella (L. ). Upon egg hatching, many teratocytes originating from the serosal membrane were released into the parasititized larval hemocoele. The present paper was focused on studying the synthesis of protein, secretion and ultrastructural changes of C. vestalis teratocytes by using the related techniques. Protein two-dimensional electrophoresis showed that the mature teratocytes of C. vestalis had strong ability of protein synthesis, the proteins with molecular sizes between 40. 98 to 94. 64 kDa were abundant. During the later stages of parasitism, the ability of protein synthesis in teratocytes declined, and the significant ultrastructural alterations of teratocytes were observed. There were sparse mitochondria and endoplasmia reticulum, and many vacuoles in cytoplasm, coalescence of microvilli on the cell surface, cellular materials discharged. Following the parasitoid's emergence, some cells underwent decomposition. In taro, the mature cells could secrete lipids into medium. At different temperatures, the teratocytes developed quickly accompanying with the development of parasitoid larvae when the temperature increased, it showed that the pattern of teratocytes grew synchronously with the development of parasitoid larvae. Cotesia vestalis (Braconidae): Parasite; Plutella xylostella (Plutellidae): Host. Chinese none